Conventionally, there has been developed a plasma generation device that generates local plasma byway of an ignition plug discharge and enlarges the plasma by way of an electromagnetic wave such as a microwave (for example, see Patent Document 1). The plasma generation device is provided with a mixing circuit that mixes energy of a high voltage pulse for the discharge and energy of an electromagnetic wave from an electromagnetic wave generation device. The mixing circuit is electrically connected to an input terminal of the ignition plug. As a result of this, the energy of the electromagnetic wave and the energy of the high voltage pulse are supplied to the ignition plug through a same transmission line (electric path). Accordingly, the ignition plug serves as both a spark discharge electrode and an antenna for electromagnetic wave emission.
However, a center electrode of an ignition plug (a whole portion of the ignition plug that forms a discharge gap with a ground electrode within the ignition plug extending from a terminal part connected with an ignition coil up to a tip end part is referred to as “the center electrode” and the same applies hereinafter) generally used in a conventional plasma generation device is usually constituted by an iron-based alloy except in the tip end portion. This means that the principal component of the center electrode is an iron having a high magnetic permeability. Accordingly, the electromagnetic wave provided from an alternating current power supply flows on a surface of the center electrode, resulting in a great power loss.
Also, Patent Document 2 discloses a technology of providing a high frequency power of between 50 kHz and 100 MHz to a center electrode of an ignition plug, thereby generating plasma between the electrodes.
More particularly, the ignition plug is provided with a tube-shaped insulator having an axial hole penetrating therethrough in an axial direction, the center electrode arranged at a tip end side of the axial hole, a terminal metal fitting arranged in the axial hole closer to a back end side than the center electrode, a main metal fitting arranged in a manner so as to surround the insulator, and a ground electrode electrically connected to the main metal fitting. The terminal metal fitting is electrically connected to the center electrode via an axis and provided with a high frequency power from outside, thereby plasma is generated between the center electrode and the ground electrode. At least apart of an inner surface of the axial hole is formed with a metal coating having electrical conductivity higher than iron and the center electrode is held in electric contact with the metal coating. Also, the terminal metal fitting is held in contact with the metal coating at a position closer to the back end side than the center electrode. In this manner, the ignition plug is provided with a first electrical path adapted to supply the electric power through the terminal metal fitting and the axis to the center electrode and a second electrical path adapted to supply the electric power through the terminal metal fitting and the metal coating to the center electrode. Accordingly, since the cross-section area of the electrical paths increases, the electrical resistance thereof decreases and the power loss is reduced.
However, the axial hole of the insulator of the ignition plug is approximately between 2 mm and 5 mm in inner diameter and approximately between 60 mm and 100 mm in length. The metal coating is required to be formed on the inner surface of the thin axial hole (for example, a paste obtained by mixing a powdered metal in an organic solvent is to be coated thereon). In addition, in order to form the two electrical paths, it is required to provide a gap between the axis and the metal coating, filling the gap with filler such as talc. Thus, there has been a problem of hard and complex manufacture.
Furthermore, the high frequency power disclosed in Patent Document 2 is assumed to be between 50 kHz and 100 MHz in frequency. In a case of a microwave having a frequency (for example, 2 GHz or higher) higher than the high frequency power, since the skin effect increases, there is a need of due consideration of both magnetic permeability and electrical conductivity. Furthermore, in the case of the ignition plug disclosed in Patent Document 2, it is not considered that the ignition plug is supplied with both a pulse voltage for a spark discharge and an electromagnetic wave provided as energy to the spark discharge.